This application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2007-065507 filed in Japan on Mar. 14, 2007, the entire contents of which are herein incorporated by reference.
The present invention relates to fixing apparatuses in which a metal roller is arranged that is idly rotated by contacting a circumferential surface of a fixing roller, which fixes a toner image that has been transferred onto a paper, and that is provided with a cleaning member for cleaning the circumferential surface of the fixing roller using the metal roller, and to image forming apparatuses equipped with these fixing apparatuses.
These image forming apparatuses are provided with fixing apparatuses that melt and fasten unfixed toner onto paper. In this type of fixing apparatus, a thermal fixing roller system is employed in which, while the paper is transported with being sandwiched in a pressing area (fixing nip portion) between a hot roller and a pressure roller, the paper is subjected to heat and pressure by the hot roller and the pressure roller so that toner on the paper is thermally melted and fixed there.
In these thermal fixing roller system fixing apparatus, the surfaces of both rollers are heated by a heat source (heater) that is arranged inside the hot roller. And toner on the paper that is being transported is melted at the fixing nip portion and fastened (fixed) onto the paper. At this time, not all the unfixed toner on the paper is fastened onto the paper, and some adheres to the pressure roller via the hot roller and the fixing nip portion. For this reason, cleaning members are arranged for the hot roller and the pressure roller respectively in order to clean “smeared toner” that is in an adhered state thereon. Of these, a metal roller that is idly rotated by contacting the circumferential surface of the pressure roller is generally used as the cleaning member for cleaning the surface of the pressure roller.
In recent years, double sided printing has become possible as part of the increasing multifunctionality of image forming apparatuses. In the case of double sided printing, since there is image information (a toner image) also on the unfixed surface (pressure roller side) of the transported paper, toner in a half-melted state adheres to the pressure roller side also due to the surface temperature of the pressure roller and the pressure that is applied at the fixing nip portion when the paper passes through the fixing nip portion in this state.
In this case, although it is possible to arrange a large-scale cleaning member (such as a web cleaning unit) as the cleaning member arranged for the pressure roller in a same manner as on the hot roller side, this would not only result in an apparatus of a larger size, but would also result in an increased load on the drive source of the apparatus, and therefore ordinarily it is common for the aforementioned metal roller to be used as the cleaning member on the pressure roller side.
Accordingly, fixing apparatuses aimed at improving the cleaning capabilities of metal rollers such as these have heretofore been proposed (for example, see JP 2001-166626A). In this fixing apparatus, the surface temperature of the pressure roller is maintained constantly at a fixed temperature (for example, 150° C.) such that smeared toner that adheres to the pressure roller is put into a melted state, thereby aiming to improve the collection efficiency of the metal roller, which is the cleaning member arranged on the circumference of the pressure roller.
In this regard, in a case where the aforementioned double sided printing is used more frequently, the amount of toner that adheres to the pressure roller also increases, and therefore when cleaning with only the metal roller, the metal roller becomes badly smeared before the metal roller replacement cycle arrives, which is set at the design stage of the apparatus, such that there are problems involving reverse movement of smeared toner to the hot roller and an increased probability that back side smearing will occur on the transported papers. Moreover, when aiming to improve the collection efficiency of the metal roller as indicated in the above-mentioned JP 2001-166626A, the metal roller replacement cycle is further shortened, and therefore occurrences such as reverse movement of smeared toner to the hot roller and back side smearing on the transported papers also become more frequent.
In this case, when smearing of the metal roller exceeds a certain fixed level in a conventional image forming apparatus, deterioration of image quality is prevented by forcibly stopping (locking) the driving of the apparatus. However, for a user, the sudden stopping of the apparatus is unexpected and it would be convenient if some notice or warning was given in advance.
As a technique for carrying out these notices and warnings, a technique has been implemented in which the metal roller replacement cycle is set using an added number of printed sheets for example, and in a case where this is set such that replacement is performed at 100,000 sheets for example, notice is given of the replacement time when the added number of print processed sheets reaches 80,000 sheets for example.
However, as mentioned above, the extent of smearing on the metal roller is greatly different between a case in which double sided printing is frequently used and a case in which it is not, and therefore a technique of giving notice by simply using an added number of printed sheets is merely a notice for reassurance and offers little reliability. Furthermore, although it is possible to issue notices and warnings by actually detecting smearing on the paper, this necessitates mechanisms and circuit configurations for detecting ink smearing on the paper and also necessitates a determination technique for determining smearing, and therefore is not a very realistic technique.
On the other hand, as shown in FIG. 11, which shows a relationship between metal roller smearing and a locking phenomenon of a fixing drive source, a load state of a fixing drive portion is such that the load gradually increases from an initial state of an installation time or a maintenance completion time of the image forming apparatus. As described earlier, a hot roller, a pressure roller, a web cleaning unit, which is the cleaning member for the hot roller side, and a metal roller, which is the cleaning member for the pressure roller side, are ordinarily arranged as rotating members in a fixing apparatus. Of these, the web cleaning unit is rotated at a same velocity as the hot roller using independent driving so that an unsoiled, fresh sheet is always supplied to the hot roller surface, and therefore a load increase at the contact area between the web cleaning unit and the hot roller is inconceivable. Furthermore, the surface of the hot roller is cleaned by the web cleaning unit and the surface of the pressure roller is also cleaned by the metal roller, and therefore as long as the cleaning is carried out reliably, there is little probability of a load increase in either of the hot roller or the pressure roller by themselves. Consequently, it is conceivable that a major cause of the load increase in the fixing drive portion is that an outer diameter of the metal roller, which is arranged at the circumference of the pressure roller and is idly rotated, increases due to collecting smeared toner, thereby resulting in an increase in the contact pressure between itself and the pressure roller. In other words, it is conceivable that the extent of smearing of the metal roller is evident as a load increase in the fixing drive portion.